Fluids may be conveyed using conduits, conveniently as a multiphase flow, in which more than one fluid is present at the same time. In oil and gas production, fluids from a well may be conveyed in a multiphase flow. This is advantageous because where more than one fluid is to be transported only one pipeline is needed. This is particularly advantageous in environments that are difficult to reach, such as sea beds and harsh climates, as the use of only one pipeline greatly reduces capital costs. Further downstream, the multiphase flow is received through a separator which separates the fluids of the multiphase flow before they are carried onward for further processing into a petroleum product.
The different fluids of a multiphase flow have different flow characteristics governed by their differing viscosities and densities. This makes it difficult to characterize multiphase flow. It is important to be able to characterize this as the flow characteristics describe the flow conditions. In turn, this is important for process control. For example, in pipe equipment one may wish to avoid excessive liquid accumulation. In separator equipment, one may wish to avoid contamination of the separator output (e.g. water into oil or oil into water). The flow characteristics may also indicate whether plugging of the flow or corrosion and erosion of pipeline and other equipment is likely. Plugging can occur by the formation of waxy deposits inside a pipeline or separator. In order to convey fluids in a multiphase flow safely, and with proper control, it is therefore imperative to have a good knowledge about the flow characteristics. One of the more important flow characteristics is the phase distribution (sometimes termed flow regime).
Flow characteristics are typically determined using empirical equations that have been tested using laboratory experiments. However, these equations are limited as they cannot take account of all of the variables that may be present in a working multiphase flow system, such as inclination of equipment, variations in flow rate and so on. Furthermore, as the test rigs used to verify the empirical equations have significantly smaller diameters than the equipment actually used in production, the validity of the models for the full scale production equipment is unknown.
Multiphase flow can also be predicted theoretically using models and equations but these suffer similar limitations to using empirical models.
Other techniques for determining flow characteristics involve using flow rates or using heat sources and associated probes.